Various current techniques for revealing concealed objects generally fall into two general categories: active and passive. There are advantages and disadvantages to both, some of which are natural constraints, due to electromagnetic phenomena, some of which are technological and practical constraints of cost and availability of electronic components, and some of which are performance related regarding the nature of the material characteristics which are revealed and concerns of public safety and preference to not be radiated with particular forms of electromagnetic energy. In all forms of detection, either naturally occurring radiation, instrument provided radiation, or some combination is employed in the detection process.
Passive thermal images, including millimeter and other wavelengths, provide benefits in practical and safe detection of concealed objects on people. In passive millimeter wave (and other wavelengths in general) imaging for concealed object detection on people, the natural radiation from the person and the generally lower temperature environment in which the person is immersed cause a reflective object on that person to reveal itself in contrast because the object reflects a lower temperature than the body radiates. Millimeter waves are located in a part of the electromagnetic spectrum which has advantages in revealing objects concealed on humans because clothing is, for the most part, transparent at these wavelengths. However, conventional imaging of passive thermal scenes, including millimeter waves, combines both reflectance and temperature in a manner which confuses object detection, especially when the object temperature nears that of the human subject.
Various methods and apparatus are currently employed for the purpose of improving contrast by providing illumination sources. Many of these techniques suffer when applied in a practical system from sensor overload and uneven illumination. All of them are designed to accomplish the goal of enhanced detection of reflective objects. They do not, however, eliminate the effects of the temperature of the object being concealed, especially when that object is not reflective.
Detection by thermal means is complicated by the fact that the apparent (sensed) temperature of any patch of the subject or concealed object is (in a simplified form) a combination of 1) the reflectivity of the patch multiplied by the temperature of the environment it is reflecting, 2) the temperature of the patch multiplied by the emissivity of the object, and 3) the transparency of the patch multiplied by the apparent temperature of the underlying object. One serious problem facing the practical use of passive (thermal) detection of concealed objects is to detect objects which are mostly low in reflectance and partially transparent. This difficulty in detection is especially so when those concealed objects are near or in contact with the human's thermal environment and therefore near or at the human skin temperature. Examples of such concealed objects are powdered drugs, documents, liquids, and explosives. In this situation (passive thermal scene) the discriminating feature between human and concealed object is reflectivity rather than apparent temperature. In general, passive thermal detection of objects warmed to near body temperature requires that the reflectivity be determined separately from the temperature. The remaining problem in passive thermal detection is to detect objects which have either low reflectance or high reflectance regardless of their temperature.
Accordingly, need exists for a system and technique in which passive thermal images may be used for practical and safe detection of concealed objects on people and which does not suffer from the above described problems.